E. I. du Pont de Nemours and Company (DuPont) has been in the business of making Tyvek.RTM. spun bonded olefin sheet product for many years. However, the commercial process for making Tyvek.RTM. includes the use of a CFC (chlorofluorocarbon) spin agent. As the use of CFC's will soon be prohibited, DuPont has been developing a non-CFC process for manufacturing Tyvek.RTM. sheet. Unfortunately, there is, as yet, no identified spin agent that may be used as a simple substitute in place of the present CFC spin agent without requiring substantial modifications of the process or process conditions for manufacturing the product.
Thus, an entirely new facility has been built to manufacture Tyvek.RTM. sheet using a substantially modified process and a very different spin agent. The new spin agent is a hydrocarbon, namely: normal pentane, and just about every process activity and condition has been changed or scrutinized because the new spin agent does not act or react exactly like the CFC spin agent in the present commercial system. It is of course, the intent of all the developmental work to be able to produce essentially the same sheet product as made in the conventional commercial process so as to continue to develop the business and markets that the Tyvek.RTM. business has created.
The developmental work for recreating the process of making Tyvek.RTM. sheet has the additional object to form improved products that have better characteristics for current and new end uses.
It is a particular object of the present invention to provide sheet products that have a wider range of Gurley Hill Porosity Values than that which is attainable by conventional nonwoven technology.
SUMMARY OF THE INVENTION
The invention is directed to a number of related sheet products made with polymeric man-made fiber that may be characterized in a number of independent ways. For example, one sheet has and opacity of at least 80 percent and a Gurley Hill Porosity Value of at least 120 seconds. Preferably this sheet product has a basis weight of less than 2.5 oz/sq yd and more preferably a basis weight of less than 1.7 oz/sq yd. Another sheet has a basis weight of at least 1.4 oz/sq yd and a Gurley Hill Porosity of less than 20 seconds. Another sheet has less than forty percent voids in the cross sectional area wherein no more than five percent have extremum lengths greater than 27 microns. A further sheet has at least thirty percent voids and at least five percent of the voids have extremum lengths greater than 23 microns.
A still further sheet is fully bonded and has a Correlation relative to spatial period wherein the correlation is in the range of 0.4 to 0.8 at a 15 pixel spatial period, 0.45 to 0.85 at a ten pixel spacing period, and 0.3 to 0.8 at a 20 pixel spatial period, wherein the measurements are based on a Hewlett Packard Deskscan II scanner operating under standard conditions and the pixels are approximately 169 microns square. Another sheet is similarly characterized but having a correlation of 0.1 to 0.5 at a 15 pixel spatial period, 0.15 to 0.55 at a ten pixel spatial period and a 0.05 to 0.45 correlation at a 20 pixel spatial period wherein the same equipment is used under normal conditions and the pixel size is the same.
A still further characterized sheet is set forth which is fully bonded and has a Haralick feature 13 Information Measure of Correlation between 0.19 and 0.35 at a ten pixel spatial period, between 0.15 and 0.325 at a 15 pixel spatial period, and between 0.125 and 0.3 at a 19 pixel spatial period wherein the pixels are approximately 169 square microns. A different sheet is similarly characterized and set forth having a Haralick feature 13 Information Measure of Correlation in the range of 0.075 to 0.2 at a ten pixel spatial period, 0.05 and 0.175 at a 15 pixel spatial period, and between 0.05 and 0.175 at a 19 pixel spatial period.
The invention further relates to a sheet being defined as a nonwoven sheet product made of overlapping layers of flash spun fibers bonded together with at least heat and pressure, wherein the web comprises fibrils having a mean apparent fiber width of greater than 24 microns, a median apparent fiber width of greater than about 13.5 microns and wherein the fibers are spun from one or more orifices at less than 100 pounds per hour per orifice, and wherein the sheet product has a Gurley Hill Porosity Value of greater than 30 seconds. An additional nonwoven sheet product is set forth which is made of overlapping layers of flashspun fibers bonded together with at least heat and pressure, wherein the web comprises fibrils having a mean apparent fiber width of less than 25 microns, a median apparent fiber width of less than about 13.5 microns, such that the fibers are spun from one or more orifices at less than 100 pounds per hour per orifice, and wherein the sheet product has a Gurley Hill Porosity Value of less than 20 seconds. A further nonwoven sheet product is set forth which is made of a plurality of overlapping plexifilamentary film-fibril webs wherein the webs have openings between the fibrils and the openings have an average perimeter of at least 2650 microns, the sheet includes portions which have at least four separate overlapping web swaths and the Gurley Hill Porosity Value is at least 25 seconds. Another nonwoven sheet product is set forth which is made of a plurality of overlapping plexifilamentary film-fibril webs wherein the webs have openings between the fibrils and the openings have an average perimeter of less than 3300 microns, the sheet includes portions which have at least four separate overlapping web swaths and the Gurley Hill Porosity Value is less than 75 seconds.
The invention is further related to a nonwoven sheet product made from a plurality of overlapping plexifilamentary film-fibril webs, wherein the sheet product has a cross section comprising fibrils which are bonded together and form voids within the sheet, the voids forming less than forty percent (40%) of the cross sectional area of the sheet and wherein the voids have a general shape so as to appear long and thin and wherein no more than five percent of the voids have extremum lengths greater than 27 microns.
Preferably, the nonwoven sheet product has an opacity of greater than 80. More preferably, the nonwoven sheet product according to claim 18 wherein the Gurley Hill Porosity Value is greater than 80. In addition, it is preferred that the nonwoven sheet product has less than fifteen percent of the voids having extremums greater than four microns.
The invention also relates to a method of characterizing a plexifilamentary film-fibril web comprising a number of steps, in particular, the first step is scanning a sample of the plexifilamentary film-fibril web with optical scanning equipment to create an image of the scanned sample and the next step is to digitize the image of the scanned sample. Thereafter, the openings between fibrils in the digitized image are identified and the perimeters of the openings between the fibrils to are measured to create a data set for comparison to other web samples.
The invention further relates to another method of characterizing a plexifilamentary film-fibril web comprising scanning a sample of the plexifilamentary film-fibril web with optical scanning equipment to create an image of the scanned sample and digitizing the image of the scanned sample. Thereafter, the individual fibrils in the digitized image are identified and the width of the fibrils are measured to create a data set for comparison to other web samples.
Finally, the invention relates to an additional method of characterizing a sheet material comprising the steps of cutting a sample of the sheet material to reveal a cross section thereof, scanning the cross section of the sample of the sheet material with a scanning electron microscope to create an image of the scanned sample and digitizing the image of the scanned sample. Thereafter, the voids in the cross section in the digitized image are identified and the voids are measured to create a data set for comparison to other sheet samples.